Advisories

WDPN31 PGTW 131500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 16W (NANGKA) WARNING NR 
005//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 28.0N 151.0E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 555 NM EAST-NORTHEAST OF IWO TO
   MOVEMENT PAST 6 HOURS: EAST-NORTHEASTWARD AT 13 KTS
   SIGNIFICANT WAVE HEIGHT: 13 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TROPICAL STORM (TS) 16W APPEARS TO HAVE UNDERGONE A RAPID VORTEX
DECOUPLING EVENT OVER THE PAST SIX HOURS. ANIMATED SHORTWAVE
INFRARED (SWIR) SATELLITE IMAGERY DEPICTS A PARTIALLY EXPOSED LOW
LEVEL CIRCULATION CENTER (LLCC) TO THE SOUTHWEST OF AN AREA OF
WANING CONVECTION. HOWEVER, THE LLCC REMAINS MOSTLY OBSCURED AND
CHALLENGING TO LOCATE WITH HIGH CONFIDENCE USING THE SWIR ALONE.
THE TIMELY COLLECTION OF TWO ASCAT PASSES, AT 131034Z AND 131011Z,
PROVIDED CONFIRMATION OF THE VORTEX DECOUPLING, DEPICTING A SLIGHTLY
ELONGATED LLCC WELL SOUTH OF THE EXPECTED POSITION. THE ASCAT SHOWS
35 KNOT WINDS ALONG THE WESTERN FLANK AS WELL AS IN THE NORTHEASTERN 
QUADRANT. THE INITIAL POSITION IS ASSESSED WITH HIGH CONFIDENCE BASED 
ON THE MULTIPLE ASCAT PASSES. THE INITIAL INTENSITY IS ASSESSED WITH 
MEDIUM CONFIDENCE, CONSISTENT WITH THE MULTI-AGENCY DVORAK CURRENT 
INTENSITY ESTIMATES AND THE CIMSS ADT ESTIMATE. THE ENVIRONMENT IS 
NOW UNFAVORABLE FOR TROPICAL CYCLONE DEVELOPMENT, WITH HIGH 
SOUTHWESTERLY VWS, AND DRY AIR ENTRAINMENT OFFSETTING THE STRONG 
POLEWARD OUTFLOW AND WARM SSTS.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: NORTHERN FLANK OF A LOW- TO MID-LEVEL
NEAR EQUATORIAL RIDGE (NER) TO THE SOUTH

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   RJTD: T2.5 - 35 KTS
   RCTP: T2.5 - 35 KTS
   KNES: T2.5 - 35 KTS
   CIMSS SATCON: 49 KTS AT 131210Z
   CIMSS ADT: 34 KTS AT 131210Z
   CIMSS AIDT: 43 KTS AT 131210Z
   CIMSS D-MINT: 45 KTS AT 131201Z
   CIMSS D-PRINT: 46 KTS AT 131210Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: UNFAVORABLE
   VWS: 30-35 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG POLEWARD
   OTHER FACTORS: DRY AIR ENTRAINMENT ALONG THE WESTERN FLANK. 

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: HIGH
   INITIAL WIND RADII: HIGH

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING.

FORECAST DISCUSSION: TROPICAL STORM (TS) 16W IS FORECAST TO TRACK
NORTHEASTWARD THROUGH THE FORECAST PERIOD, WHILE STEADILY
DECELERATING. THE TRACK FORECAST REMAINS SUBJECT TO A HIGH DEGREE
OF UNCERTAINTY HOWEVER, AS MODEL GUIDANCE CONTINUES TO DEPICT A
BIFURCATION BETWEEN THE PHYSICS-BASED MODELS AND THE AI-BASED
MODELS. CONSISTENT WITH THE EARLIER MODEL RUNS, THE MAJORITY OF THE
PHYSICS MODELS (ALL BUT ECMWF) DEPICT A SHARP TURN NORTHWESTWARD
AFTER TAU 12, WHILE THE AI MODELS PLUS THE ECMWF AND ECEPS MEAN,
DEPICT AN EAST-NORTHEASTWARD TRAJECTORY CONTINUING THROUGH THE END
OF THE FORECAST. THE BIFURCATION IN THE MODEL TRACKERS IS BEING
DRIVEN BY DIFFERENCES IN HOW THE MODELS HANDLE THE STEERING
PATTERN, COMPLICATED NOW BY THE FACT THAT THE VORTEX IS RAPIDLY
DECOUPLING, WHICH WILL CHANGE THE LEVEL AT WHICH THE SYSTEM IS
STEERED. NOW THAT THE VORTEX IS DECOUPLING, THE STEERING LEVEL WILL
LOWER, WHERE THE OVERALL PATTERN IS MORE STABLE. THE NER TO THE
SOUTH WILL REMAIN THE DOMINANT STEERING INFLUENCE FOR THE FIRST
18-24 HOURS OF THE FORECAST. THEN THE SYSTEM SLOWS SHARPLY AS IT
MOVES INTO A WEAK STEERING ENVIRONMENT AS THE NER RECEDES WESTWARD
AND IS REPLACED BY AN EXTENSION OF THE STR CENTERED FAR TO THE
EAST. THIS WILL EVENTUALLY TURN THE SYSTEM MORE NORTHWARD, WHICH IS
REFLECTED BY A SHALLOW TURN POLEWARD BY THE END OF THE FORECAST.
REGARDING INTENSITY, INCREASING SHEAR AND DRY AIR ENTRAINMENT WILL
HASTEN THE VORTEX DECOUPLING, RESULTING IN A SLOW WEAKENING TREND
THROUGH TAU 48, WHEN THE SYSTEM WILL DISSIPATE. 

MODEL DISCUSSION: TRACK GUIDANCE CONTINUES TO BE HIGHLY UNCERTAIN,
WITH A BIFURCATED APPEARANCE. THE NAVGEM IS THE WESTERN-MOST OF THE
PHYSICS MODELS, WHILE THE GOOGLE DEEPMIND IS THE EASTERN-MOST,
RESULTING IN A 500NM CROSS-TRACK SPREAD BETWEEN OUTLIERS AT TAU 48.
THE ECMWF AND ECEPS MEAN LIE ON THE NORTHERN EDGE OF THE AI-BASED
MODEL TRACKERS AND THE GFS HAS SHIFTED EASTWARD SLIGHTLY AND IS
POSITIONED ON THE EASTERN SIDE OF THE PHYSICS-BASED MODELS. ENSEMBLE 
GUIDANCE CONTINUES TO DEPICT A VERY WIDE MEMBER DISPERSION, ACROSS 
ROUGHLY 1000NM FROM THE COAST OF JAPAN TO WAKE ISLAND. HOWEVER, TRACK 
DENSITY ANALYSIS OF THE JTWC EXPERIMENTAL SUPER ENSEMBLE SUGGESTS MORE 
MEMBERS SUPPORT THE EASTWARD TRACK, LENDING INCREASED CONFIDENCE TO 
THE JTWC FORECAST TRACK, WHICH IS PLACED NEAR THE ECMWF AND ECEPS 
TRACKERS, WITH MEDIUM CONFIDENCE. THE MAJORITY OF THE INTENSITY 
GUIDANCE MAINTAINS THE SYSTEM AT ROUGHLY 30-35 KNOTS FOR THE DURATION 
OF THE MODEL RUN (OUT TO 120 HOURS), WITH THE EXCEPTION OF THE COTC 
AND THE GOOGLE DEEPMIND WHICH BOTH DISSIPATE THE SYSTEM WITHIN 48 
HOURS. THE JTWC FORECAST FITS CLOSEST TO THESE MODELS, AND DEVIATES 
FROM THE CONSENSUS BASED ON AN ANALYSIS OF THE OVERALL ENVIRONMENT AND 
THE ONGOING DECOUPLING EVENT. 

FORECAST CONFIDENCE:
   TRACK 00-72 HR: MEDIUM
   INTENSITY 00-72 HR: MEDIUM//
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